Stormwater runoff includes the initial runoff or “first flush” that contains sediments, oil, and other pollutants flushed from surface areas, and other runoff that can be considered essentially pollution-free. The pollution-free runoff includes the later runoff from the surface areas that generated the first flush, and runoff from areas without surface pollutants. In major storm events the volume of non-first flush is substantially greater than the volume of first flush.
Stormwater treatment systems have been developed to remove pollutants from the first flush. Conventional first flush treatment systems include systems that pass the first flush through a filter to remove pollutants. The filter can be a relatively inexpensive low-head filter because of the relatively low volume and flow of runoff to be filtered.
Underground stormwater management systems have also been developed that receive runoff at a high rate during a major storm event, and discharge the runoff at a lower rate to a storm drain. Such systems include an underground storage chamber that receives and stores the water that accumulates while the flow into the storage chamber is greater than the discharge out. The water discharges at a relatively high head from the storage chamber to enable discharge near the maximum discharge rate allowed by applicable law or regulation. The discharge is normally not filtered, but if filtering is desired an expensive high-head filter must be used because of the high volume and flow of runoff being filtered.
Stormtech LLC, Wethersfield, Conn., markets an underground stormwater management system that includes an open-bottomed chamber that receives the first flush. The chamber is placed on stone base. A woven geotextile sheet is placed between the chamber and the stone base. The geotextile filters sediment from the first flush as the first flush leaves the chamber through the stone base.
Accumulated sediment in the chamber must be periodically removed. Access to the chamber is provided by a manhole located on an end of the chamber. A high-pressure water spray is used to flush the sediment off the geotextile sheet and back into the manhole for vacuuming.
An underground stormwater management system that captures first flush in a chamber and filters sediment from the first flush leaving the chamber but without the need for a high-pressure water spray to remove the accumulated sediment is desired.